1. Field of the Invention
This invention relates to a method of transferring toner powder images by pressure and an apparatus therefor, and more particularly to a method and an apparatus for use in transfer-type copying machines, electrostatic printers, etc. for pressure-transferring an electrostatic image developed with a toner powder.
2. Description of the Prior Art
With copying machines of the transfer type, a uniformly charged photoconductive member is exposed to the optical image of an original to form thereon a latent electrostatic image, which is developed and then transferred onto plain paper. With electrostatic printers, the desired latent electrostatic image is formed on a dielectric member by an electrostatic recording head, and the image is developed and thereafter transferred onto paper. The apparatus of either type usually use a two-component toner for development, such that the toner is charged to a polarity opposite to that of the latent image and then deposited on the image to obtain a toner image, which is subsequently transferred onto paper electrostatically.
The electrostatic transfer of the toner image is effected most generally by applying a voltage of opposite polarity to the toner to the rear side of the transfer paper by a corona charger or electroconductive transfer roller. This method, however, requires a large power supply means and has the drawback that copy images are not always available with high transfer efficiency and stability depending on the ambient conditions, especially humidity.
In recent years, transfer-type copying machines have been introduced into use which use a mono-component toner having magnetic and electroconductive properties, because difficulties are encountered with two-component toners in maintaining the toner and the carrier in a constant ratio and also because the carrier is liable to cling to the latent image. A copying machine of this type, still resorting to electrostatic transfer, fails to assure a stable transfer operation. Additionally, the toner, which has no polarity in itself, requires a very complex electrostatic transfer system.
In view of the foregoing situation, a method has been proposed as in U.S. Pat. No. 3,937,571 in which toner images are transferred by pressure without using electrostatic attraction and which is almost free of the influence due to the ambient conditions. According to the pressure transfer method, a transfer material is passed between the surface of a toner image bearing member and a pressure roller pressed against the surface under a predetermined pressure. Although this method is almost unaffected by variations in the ambient conditions as mentioned above and can be practiced with a simple apparatus without requiring any electrical means, the method is considerably lower in transfer efficiency than usual electrostatic transfer methods and is unable to afford satisfactory copy images. To overcome the drawback, it appears useful to set the pressure roller at a higher pressure, but the toner image bearing member will then have a seriously shortened life, especially when the bearing member is a photoconductive member. Alternatively a toner may be used which can be transferred properly at a low pressure. However, this requires complicated transfer conditions and also poses a great limitation on the kinds of toners usable.